The present invention relates to medical imaging systems in general, and ultrasound imaging systems in particular.
In diagnostic medical imaging, it is often necessary to view the same tissue sample at different times. For example, in stress tests, a physician may compare recorded images of the heart at rest to real time images of the heart under stress in order to detect the presence of disease. Similarly, in order to detect any growth of a tumor it is useful to compare images of the tumor taken at various time intervals.
Ultrasound is becoming an increasingly common technique for non-invasively imaging body tissue or blood flow. In order to obtain similar images of a tissue sample, it is necessary that the ultrasound probe that transmits ultrasonic signals into the body and receives corresponding echo signals be oriented in substantially the same direction for each image. In the past, a physician or sonographer would typically make a handwritten note in a file concerning the position of a transducer. This would be used as a guide to placing the transducer the next time an image of a tissue sample is to be obtained. Alternatively, if subsequent images are to be taken in a relatively short amount of time, the physician may outline the position of the transducer on the patient with a felt-tip pen. Both these techniques allow only rudimentary alignment of the transducer and therefore can make it difficult to compare images of the tissue sample that are taken at different points in time.
To improve the ability of a physician or sonographer to duplicate the orientation of an ultrasonic transducer, a reference image is obtained and stored on a recordable medium. The reference image contains a tissue sample that is to be compared with later acquired images of the tissue sample. To orient the transducer in the same direction used to obtain the reference image, subsequent images are compared against the reference image. Results of the comparison are fed back to the user so that the transducer position can be moved to obtain a higher degree of similarity with the reference image.
In one embodiment of the invention, the comparison is performed as a sum of absolute differences wherein the intensity of each pixel in the reference image is subtracted from the intensity of a corresponding pixel in a subsequent image. The sum over the entire set of pixels in the reference and subsequent image is used to provide feedback to the user so that the user knows if the transducer orientation is becoming more or less like the orientation used to obtain the reference image. Once a subsequent image is obtained having a high degree of similarity, the subsequent image is stored and compared against the reference image so that a physician or sonographer may determine how the tissue sample changes over time or with stress, etc.